Thin carbon coating of optical waveguides

ABSTRACT

An optical waveguide fiber having a thin carbon coat over the clad glass layer is disclosed. The carbon coated waveguide shows superior dynamic fatigue resistance, improved adhesion of a polymer overcoat in environmental testing, excellent attenuation stability in environmental testing, and easy colorability.

This application is a 371 of PCT/U.S.98/11345 filed Jun. 3, 1998 whichclaims benefit of Provisional. No. 60/050,551 filed Jun. 23, 1997.

BACKGROUND OF THE INVENTION

The invention is directed to a thin carbon coating on an opticalwaveguide fiber. The coating acts to improve the waveguide fiberperformance. More particularly, a thin carbon coating, formed on theclad glass layer of the waveguide fiber, has been found to improvedynamic fatigue performance of the waveguide fiber. In addition, thecarbon coating markedly improves resistance to delamination between thepolymeric coating and the waveguide fiber, under severe environmentalconditions such as immersion in water.

The concept of coating optical waveguide fibers is known in the art.Polymer coatings have been developed to protect the waveguide fiber fromhandling damage as well as to reduce the impact of bending on waveguideattenuation. Also, hermetic coatings have been developed to seal thewaveguide fiber from OH— ions, which enable growth of waveguide surfaceflaws when the waveguide is under stress. A hermetic coating also isimportant in protecting the waveguide from corrosive materials, andgasses, particularly hydrogen, which can diffuse into the waveguide andcause increases in attenuation.

Of the several types of coating material tested in the search for ahermetic coating, carbon has been found to be most compatible with themanufacture, packaging and use of a waveguide fiber.

The thickness of the carbon layer sufficient to provide hermeticity hasbeen found to be in the range of 1000° A or greater. In U.S. Pat. No.4,964,694, Oohashi et al., carbon coating thickness of the range of 1000to 6000° A is taught ( col. 3, II. 29-34). Thickness less than 1000° Atend to allow pinhole formation in the coating. Thickness greater than6000° A tend to crack and peel from the waveguide surface. Hermeticityis also measured in terms of resistance to the passage of hydrogenthrough the coating. See, for example, U.S. Pat. No. 5,000,541,DeMarcello et al., col. 4, II. 19-39. At col. 5, II. 11-15, of '541DeMarcello, a carbon layer of thickness of 1000° A is noted as providinga barrier to the diffusion of hydrogen.

The manufacturing and cost penalties which arise from the incorporationof a carbon coating step into the waveguide fiber manufacturing processare:

drawing speed is limited by the requirements of carbon coating thicknessand integrity;

an additional on line measurement of carbon coating thickness must beadded to the draw feedback control loop;

additional quality control testing for hermeticity must be done; and,

the black color of the waveguide complicates the process of coloring thepolymer layer to color code multiple fiber assemblies.

SUMMARY OF THE INVENTION

The invention overcomes the drawbacks of achieving hermeticity whilemaintaining some of the benefits thereof. Additional unexpected benefitsalso derive from the presence of the thin carbon coating.

Thus, a first aspect of the invention is an optical waveguide fibercoated with a carbon layer having a thickness no greater than about 100°A. It is contemplated that thickness no greater than 50° A aresufficient. As carbon coating becomes thinner, one may expect thewaveguide properties to approach those of a non-carbon coated waveguidefiber. Some benefit in terms of carbon coated waveguide fiberperformance may be expected at thickness about 10 μm. The thin carbonlayer is distinguished from a hermetic carbon coating by itspermeability to fluids, such as hydrogen. However, the dynamic fatigueconstant, which is about 20 for a silica clad waveguide, is greater thanabout 25 in the case of a waveguide having a thin carbon layer. Thisincrease is quite significant in light of the fact that the fatigueconstant appears as an exponent in the equation predictive of time tofailure.

In addition to the characterization of the thin carbon layer by itsthickness, the layer may also be characterized by its resistance perunit length, which is no greater than about 4 Mega-ohms/cm (MΩ/cm). Thethin layer of carbon is bonded to the waveguide clad glass layer. Thelayer is colored a light gray.

A second aspect of the invention is the surprising discovery that thethin carbon layer acts to essentially prevent delamination of thepolymer coating. The integrity of the waveguide fiber having aprotective polymer coating is such that substantially no attenuationincrease was induced by immersing the carbon and polymer coatedwaveguide in water for extended time periods. The standard environmentaltests call for room temperature water soak and hot water soak, about 65°C., for 30 days. The tests on the novel carbon coated waveguide fiberwere extended to 128 days, in both room temperature and hot water, andstill substantially no induced attenuation was observed.

An additional benefit of the coating results from its light gray colorwhich allows, in contrast to the black hermetic coating, the waveguidefiber to be color coded using methods and pigments known in the art. Thecolors successfully applied and tested were yellow, white, red, andgreen. These colors are believed to be the most difficult to apply andthe most likely to change in environmental testing.

BRIEF SUMMARY OF THE DRAWINGS

FIG. 1 is an end view of an optical waveguide fiber having a thin carboncoating and a polymer coating.

FIG. 2 is a Weibull strength chart showing failure probability vs.applied stress.

FIG. 3 is a chart of attenuation vs. time for a waveguide in anenvironmental test.

DETAILED DESCRIPTION OF THE INVENTION

The optical waveguide fiber having the novel thin carbon coating ischaracterized by:

a dynamic fatigue constant greater than about 25;

superior polymer coating adhesion in severe environments; and,

ease of coloring similar to that of a polymer coated waveguide having nocarbon layer. Thus, a strength benefit of a carbon coated waveguide isrealized, while essentially none of the drawbacks associated with ahermetic coating need be dealt with. In addition to the ease ofcoloring, it is believed the thin carbon coat may be applied at higherdraw speeds than that of a hermetic coating process. No additional online measurements coupled to the draw control loop are required andquality control can be maintained by making a statistically significantnumber of off line measurements of coating electrical resistance. Thesestatements are based upon the results, discussed below, which show thatresistance per unit length of the order of mega-ohms provide a suitablethin coating. In sharp contrast, the resistance requirement for ahermetic carbon coating, i.e., a coating having a thickness no less thanabout 500° A, is in the kilo-ohm range, three orders of magnitude lower.

The end view illustration of the novel waveguide is shown in FIG. 1. Theclad glass layer 2 is surrounded by and adhered to thin carbon layer 4.The outer layer 6 represents the protective polymer coating, which maycomprise one or more layers. Note that the carbon layer is formeddirectly onto the glass surface of the waveguide fiber.

A method of forming the coating comprises pyrolytic deposition of carbononto the waveguide fiber as the fiber emerges from the hot zone of thedraw furnace. The fiber passes from the hot zone into a controlledenvironment chamber where a carbon containing compound reacts to producea carbon layer on the waveguide surface. The reaction may be driven bythe heat from the waveguide fiber. A process suitable for applying ahermetic coating or the thin carbon coating of this application is foundin U.S. Pat. No. 5,346,520, Meabon, et al ('520 patent).

Because the concentration of the carbon containing compound in thereactor is low in the thin coating process, the pyrolytic reactiontended to be somewhat unstable. The pyrolytic reaction was stabilized byintroducing a relatively inert gas into the flow. A gas such as argonwas used. Depending upon the thickness of the carbon coated layer, theflow rate of the argon was in the range of 0 to 75% by volume of thetotal flow of gas into the reactor vessel.

EXAMPLE Strength Testing of Carbon Coated Waveguides

A waveguide fibers was prepared having a thin carbon coating on the cladglass surface. A polymer coating was applied over the carbon coating.The waveguide was strength tested to determine a Weibull strengthdistribution and a dynamic fatigue constant.

The Weibull plots shown in FIG. 2 show the failure probability of thefiber versus applied stress. The steep slope, straight line appearanceof the plots is markedly similar to those characteristic of hermeticcoated waveguides.

The data was generated by applying linear tension to break the fiber in20 meter gauge lengths. The environment was controlled to a temperatureof 30° C. and a relative humidity of 100%. Curve 10 is the failureprobability vs. stress using a strain rate of 0.004%/min. Curve 8represents a strain rate of 4.0%/min. The shift to the right of thehigher strain rate curve is expected because the higher rate does notallow time for certain of the waveguide surface flaws to grow tofailure. In effect, the higher strain rate acts upon a smallerdistribution of flaws, i.e. faster growing flaws.

The dynamic fatigue constant was determined by fitting a line on a chartof break strength vs. stress rate, Multiple readings of strength atfailure were taken at each of the two stress rates and n_(d), thedynamic fatigue constant, was found by fitting a line to the data. Themethod is known in the art and detailed in Fiber Optic Test Procedure(FOTP) 76, published by a U.S. standards group.

TABLE 1 Sample Gauge (m) Humidity n_(d) Resistance A 20 100% RH 27.5 3.8M□/cm A 20 100% RH 27.5 3.8 M□/cm A 0.5  50% RH 23.3 3.8 M□/cm A 0.5 50% RH 26.7 3.8 M□/cm A 0.5  50% RH 34.0 3.8 M□/cm

The 20 meter gauge test is more reliable than the 0.5 meter gauge test.It is not unusual for the shorter gauge test to yield a lower value ofn_(d). However, the data point which gives an n_(d) of 23.3 may indicatethat the carbon coating resistance of about 4 M/cm is near the limit ofhow thin the carbon coating may be. To test this hypothesis, the testingof three additional fibers was carried out at the shorter gauge length.The data is significant and does show the carbon coating is effective toincrease n_(d) to about 25 as compared to a waveguide having no carboncoating for which n_(d) is typically about 20. By comparison, a thicker,hermetic carbon coating provides an n value of 200 or greater.

Comparative Example Additional Strength Testing

A second waveguide fiber having a thin carbon layer on the clad glasssurface was prepared. In this case the electrical resistance per unitlength was 1.28 M/cm, about a factor of three lower than the previousexample, indicative of a thicker carbon layer. The data is given inTable 2.

TABLE 2 Sample Gauge (m) Humidity n_(d) Resistance B 0.5 50% RH 26.71.28 M□/cm

The data again shows the effectiveness of the thin carbon layer ingreatly improving the fatigue constant. The two data sets taken togethersuggest that a target thickness of about 4 M/cm may be appropriate.

Turning now to the effect of the carbon layer on polymer coatingadhesion, it is noted that both of the waveguide fibers described in theexamples performed well. Table 3 shows the test waveguides hadessentially no degradation in coating adhesion or waveguide functionunder severe environmental testing.

TABLE 3 Sample Environ't Time Delamin'n □A 1310 □A 1550 A 23° C. Water17 no days A 23° C. Water 31 no days A 23° C. Water 128  no 0.01 dB/km0.01 dB/km days A 65° C. Water 17 no A 65° C. Water 31 no A 65° C. Water128  no 0.04 dB/km 0.03 dB/km B 23° C. Water 17 no B 23° C. Water 31 noB 23° C. Water 128  no 0.03 dB/km 0.01 dB/km B 65° C. Water 17 no B 65°C. Water 30 no B 65° C. Water 128  no 0.03 dB/km 0.01 dB/km

The absence of delamination of the coating from the carbon coatedwaveguide is unusual. More unusual is the very small change inattenuation of the waveguide in these severe environments. The resultsof the testing of the B samples are of particular import. The B sampleshad no adhesion promoter, so that the lack of delamination is quiteunusual and unexpected. Such a coating applied to a silica surface wouldhave delaminated very quickly, i.e., in no more than a few hours.Coating delamination causes strength degradation as well as increasedattenuation. A typical environmental testing data set is shown chartedin FIG. 3. The chart is a plot of attenuation vs. time for waveguidefiber A immersed in 65° C. water. Curve 12 shows the essentiallycontinuous data readout of waveguide A attenuation at 1310 nm over the128 day time period. Curve 14 is a plot of 1550 nm attenuation forwaveguide A. The small attenuation increase is, for essentially allapplications, not sufficient to degrade performance of a systemcomprised of this waveguide fiber.

The required thickness of the novel carbon coating may be determined by:

direct measurement made on a waveguide fiber end;

measurement of electrical resistance or another electrical propertyrelated to carbon thickness;

color of the carbon coated waveguide.

This last characteristic affords another benefit of the novel thincarbon coating. Hermetic coated fiber requires a thicker carbon layerand thus appears black. The polymer coating may be somewhat transparentso that a color added to the polymer coat may be changed in appearanceby the underlying black layer. In point of fact, considerable difficultyhas been encountered in manufacture of yellow, white, green, and redpolymer coated hermetic fibers because of the black layer.

The light gray color of the carbon coated waveguide fiber disclosedherein does not interfere with the color added or applied to thepolymer. Furthermore, the colors remain within specification, asdetermined by a standard Muncell color chart, when subjected to standardenvironmental testing.

Although particular embodiments of the invention have herein beendisclosed and described, the invention is nonetheless limited only bythe following claims.

I claim:
 1. A coated optical waveguide fiber comprising; an opticalwaveguide fiber having an outer surface, wherein said outer surface hasa first coating comprising carbon, said first coating having a thicknessno greater than about 100° A, and wherein the optical waveguide fiberhas a dynamic fatigue constant ≧25, and at least one polymer coatingsurrounding and in contact with said first coating.
 2. The coatedoptical waveguide fiber of claim 1 wherein said carbon coating has anelectrical resistance per centimeter of waveguide length no greater thanabout 4 MΩ/cm.
 3. The coated optical waveguide fiber of claim 2 whereinthe electrical resistance per cm of said carbon coating is no greaterthan about 2.5 MΩ/cm.
 4. A coated optical waveguide fiber, comprising:an optical waveguide fiber having an outer surface, wherein said outersurface has a first coating comprising a layer of carbon, having athickness no greater than about 100° A, and at least one additionalcoating layer comprising a polymer, surrounding an in contact with saidthin carbon layer, and, wherein said layer comprising carbon remains incontact with said surrounding polymer layer when immersed for at least30 days in water, having a temperature in the range of bout 20 to 70° C.5. The optical waveguide fiber of claim 4 further comprising coloringagents in said polymer coating.
 6. The optical waveguide of claim 5wherein the color agents produce waveguide fibers having one of thecolors yellow, white, green, and red.
 7. The coated optical waveguidefiber of claim 4 wherein the change in optical attenuation of thewaveguide during and after water immersion is no greater than about 0.04dB/km at 1310 nm and about 0.03 dB/km at 1550 nm.